1. Field of the Invention
This invention relates to polymeric materials having useful hole transport properties. The polymers can also be electroluminescent. The invention further relates to electronic devices in which the active layer includes such polymeric materials.
2. Description of the Related Art
Organic electronic devices that emit light, such as light-emitting diodes that make up displays, are present in many different kinds of electronic equipment. In such devices, an organic active layer is sandwiched between two electrical contact layers. At least one of the electrical contact layers is light-transmitting so that light can pass through the electrical contact layer. The organic active layer emits light through the light-transmitting electrical contact layer upon application of electricity across the electrical contact layers.
It is well known to use organic electroluminescent compounds as the active component in light-emitting diodes. Simple organic molecules such as anthracene, thiadiazole derivatives, and coumarin derivatives are known to show electroluminescence. Semiconductive conjugated polymers have also been used as electroluminescent components. Polymeric materials with stilbenyl or oxadiazole side chains have been reported by Holmes et al., U.S. Pat. No. 5,653,914.
Many electroluminescent materials have poor charge transport properties. To improve these properties additional charge transport materials can be added to the light-emitting layer, or as a separate layer between the light-emitting layer and an electrode. Hole transport materials have frequently been employed. Known hole transport materials include simple molecules such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and bis[4-(N,N-diethylamino)-2-methylphenyl](4-methylphenyl)methane (MPMP), and polymeric materials such as polyvinylcarbazole (PVK), (phenylmethyl)polysilane, poly(3,4-ethylenedioxythiophene) (PEDOT), and polyaniline (PANI). It is also known to use electron and hole transporting materials such as 4,4′-N,N′-dicarbazole biphenyl (BCP); or light-emitting materials with good electron and hole transport properties, such as chelated oxinoid compounds, such as tris(8-hydroxyquinolato)aluminum (Alq3). Fused aromatic ring compounds such as pentacene are known to be electron transport materials. Copolymers having two different pendant aromatic groups have been disclosed as light-emitting materials in U.S. Pat. No. 6,007,928.
There is a continuing need for new hole transport materials.